1. Field of the Invention
The invention relates to film blowing equipment, and more particularly to a rotary die head for a film blowing machine for use with starch-based biodegradable material.
2. Description of the Related Art
Conventional blowing film molding includes the steps as follows. Plastic material from a front end of a screw extruder is pressed into a die head so that fluid plastic material turns into a siphonula through the die head. The siphonula is puffed up uniformly and freely to a tubular film with a big diameter by the compressed air from the bottom of the die head. The tubular film is extended longitudinally and cools down when drawn upwards. After that, the tubular film is flattened by a herringbone plate and draught by a drawing roller, finally rolled up. As the blowing film technique is increasingly mature, various plastic films have been applied in many fields, such as industry, agriculture, daily life and so on. Plastic films are polymer material products. Although the plastic film products bring people great convenience in use, improve the efficiency, and decrease the intensity of labor, a large amount of scrap films come into white pollution hard to be degraded. How to properly handle with the scrap films has become a tough problem.
To solve the above-mentioned problem, the scientific researchers have been trying to substitute the chemical plastic with natural biodegradable materials having high starch content. Multiple starch films which can be partially or completely biodegraded have been developed. The biodegradable film with high starch content can be melted by the moisture in the surroundings quickly and broken down by the microorganism in the soil fast, even can be transformed into fertilizer, which can ease the pollution problem caused by wasted plastic films to a great extent. And it possesses broad market prospect when people more and more care about the environment today. However, inherent characteristics of the starch-based biodegradable materials bring a new problem that cannot be manufactured in batches.
There are reasons as follows. First, compared with common high density polyethylene and low density polyethylene, starch-based biodegradable material has high viscosity and its fluidity is poor, which not only easily causes a nonuniform mixing and plasticizing, but also increases the pressure in the die head dramatically so that the conventional die heads are hard to mould the starch-based biodegradable material exactly. Second, the starch-based biodegradable material has a big shear stress, and the frictional heating during mixing and plasticizing will increase greatly. Under the condition, the conventional die heads cannot control the processing temperature accurately. Third, the starch-based biodegradable material is sensitive to the processing temperature. Excessive self-heating produced in processing will burn the material and change its properties. Thus, the processing temperature of the materials should be controlled strictly, which cannot be achieved by conventional die heads.
By this token, the blowing film principle of the starch-based biodegradable material is essentially the same as that of common plastic, but prior die heads of the film blowing machines cannot solve the problems caused by the new material due to its high viscosity, poor fluidity and sensitivity to the processing temperature. If starch-based biodegradable films is produced with the prior blowing film product line, the output and the quality thereof will be low, which cannot meet the requirements for mass industrial production.